Urea synthesis process

ABSTRACT

Aqueous urea solution formed by synthesis from ammonia and carbon dioxide is heated for water vapor evaporation and concentration by indirect heat exchange with a reacting mixture of off-gas and aqueous absorbent solution, which generates heat due to re-absorption of off-gas components in the aqueous absorbent solution.

States Patent Karafian ,e,2so

1 June6,1972

[54] UREA SYNTHESIS PROCESS 21 Appl. No.: 851,327

[52] US. Cl. ....260/555 A, 23/193, 260/534 R [51] Int. Cl l ..C07c127/04 [58] Field of Search ..260/555 A, 534R [56] References CitedUNITED sTATEs PATENTS 3,317,60l 5/1967 Otsuka et al. ..260/555 A3,506,710 4/1970 lnoue et al. ..260/555 A 3,248,425 4/1966 Ledergerber...,260/555 A 3,172,911 3/1965 ,Mavrovic ..260/555 A PrimaryExaminer-Leon Zitver Assistant Examiner-Michael W. Glynn Att0rneyJ. L.Chaboty [57] ABSTRACT Aqueous urea solution formed by synthesis fromammonia and carbon dioxide is heated for water vapor evaporation andconcentration by indirect heat exchange with a reacting mixture ofoff-gas and aqueous absorbent solution, which generates heat due tore-absorption of off-gas components in the aqueous absorbent solution.

13 Claims, 1 Drawing Figure PAIENTEDJun s 1912 MAXIM KARAFIAN BYP'YM5206 zoouad INVENTOR.

AGENT UREA SYNTHESIS PRocFss BACKGROUND OF THE INVENTION 1. Field of theInvention The invention relates to the synthesis of urea from ammoniaand carbon dioxide at elevated temperature and pressure, in processes inwhich an off-gas containing ammonia and carbon dioxide is generated byheating the synthesis effluent stream at reduced pressure.

2. Description of the Prior Art Heat exchange improvements betweencomponent streams in urea synthesis are described in U.S. Pat. Nos.3,366,682; 3,258,486; 3147,304 and 3,137,725. Improvements in completerecycle urea synthesis processes are described in U.S. Pat. Nos.3,172,911; 3,191,916; 3,155,722; 3,155,723 and U.S. patent applicationNo. 521,921 filed Jan. 20, 1966 and issued as U.S. Pat. No. 3,527,799 onSept. 8, 1970.

SUMMARY OF THE INVENTION In the present invention, the off-gas generatedfrom the urea synthesis effluent stream by heating at reduced pressureis mixed for reabsorption and reabsorbed in an aqueous absorbentsolution prior to recycle in a combination recycle urea synthesisprocess, by a process which efficiently uses the heat generated byoff-gas absorption to heat the product aqueous urea solution and therebyevaporate water, by indirect heat exchange of the respective streams.The off-gas reabsorption in an aqueous absorbent solution serves toliberate a large proportion of heat, which is thus effectively employedby indirect heat exchange to heat the product aqueous urea solution, inorder to evolve water vapor and produce a more concentrated productsolution or melt.

The principal advantage of the present invention is that the heatliberated by reabsorption of off-gas in an aqueous absorbent solution isutilized in a more effective manner. Another advantage is thataqueousurea solution produced by the urea synthesis process is concentrated ina more efficient manner, with a substantial saving of steam previouslyemployed for this purpose. A more economical complete recycle ureasynthesis process is thereby provided.

It is an object of the present invention to provide an improved ureasynthesis process.

Another object is to provide a urea synthesis process with a reducedrequirement of heating steam.

A further object is to provide a more efficient and economical ureasynthesis process.

An additional object is to more efiiciently concentrate the aqueous'ureasolution produced by a urea synthesis process which generates a mixedofi gas stream. 7

Still another object is to reabsorb the mixed off-gas of a ureasynthesis process in an aqueous absorbent solution in a more efficientmanner, by indirect heat exchange with product aqueous urea solution, tothereby evolve water vapor and concentrate the product urea solution.

These and other objects and advantages of the present invention willbecome evident from the description which follows.

DESCRIPTION OF THE DRAWING AND PREFERRED EMBODIMENTS Referring now tothe drawing, a flowsheet of a preferred embodiment of the invention ispresented.

Carbon dioxide feed stream 1 is compressed in the compressor 2 toelevated pressure and passed via stream 3 into urea synthesis autoclave4. Makeup ammonia stream 5 and recycle liquid ammonia stream 6 arecompressed in pump 7 to elevated pressure, and the resulting combinedammonia feed stream 8 is passed into autoclave 4. A recycle aqueousammoniacal ammonium carbamate stream 9 is compressed or pressurized bypump 10, and the resulting high pressure aqueous ammonium carbamatesolution stream 11 is also passed into autoclave 4. Urea synthesisreaction conditions are maintained in unit 4 to promote ammoniumcarbamate formation and dehydration to yield urea. These urea synthesisconditions in unit 4 generally consist of an elevated pressure typicallyin the range of 150 to 300 kg./sq.cm., an elevated temperature generallyin the range of 150 to 250 C, and an overall ammonia to carbon dioxidemolar ratio typically in the range of 2.5:1 to about 6:1. Under theseconditions and in the presence of excess ammonia, all of the feed carbondioxide is converted to ammonium carbamate and a major portion of thetotal ammonium carbamate present in autoclave 4 is dehydrated to urea.The resulting urea synthesis effluent stream 12 discharged from unit 4contains urea, unconverted ammonium carbamate, excess ammonia and water.

Stream 12 is passed through pressure reducing valve 13, and theresulting process stream 14, now at a reduced pressure typically in therange of 10 to 30 kg./sq.cm., is passed into the middle section ofstripping column 15, in which ammonium carbamate is decomposed andstripped from the liquid phase. An off-gas component stream rich inammonia is spontaneously evolved and separated from the residual liquidphase in the middle section of unit 15, and the liquid phase, now at areduced temperature generally in the range of to 150 C., collects abovethe partition or baffle 16 within unit 15. The liquid phase, nowprincipally containing urea, ammonium carbamate and water, is passed viastream 17 onto the stripping trays 18, and the liquid stream 17 flowsdownwards across and through the trays 18 countercurrent to a rising hotgaseous phase which decomposes ammonium carbamate and strips theresultant ammonia and carbon dioxide from the liquid phase.

The downflowing liquid discharged from trays 18, now of diminishedammonium carbamate content, is recovered on retention plate 19 whichcollects the downflowing liquid while permitting upwards gas or vaporflow. The liquid is removed from above plate 19 via stream 20, which isheated in heat exchange unit 21 by indirect heat exchange with steam orother suitable hot fluid to attain further ammonium carbamatedecomposition. The resultant heated stream 22, now at a temperaturegenerally in the range of to 200 C., and containing an evolved gaseousphase consisting of ammonia, carbon dioxide and water vapor, is passedinto the bottom portion of unit 15 above partition 23. The hot gaseousportion of stream 22 separates from the liquid phase in the bottom ofunit 15, and the hot gaseous phase rises through plate 19 and trays 18,thus serving as a stripping medium for stripping ammonium carbamate fromthe liquid on trays 18.

The liquid phase collected above the bottom partition 23 of unit 15 isremoved via stream 24, which is passed through pressure reducing valve25. The resulting process stream 26, now at a reduced pressure typicallyin the range of 2 to 10 kg./sq.cm. and a reduced temperature generallyin the range of 90 to 130 C., and containing an evolved gaseous phase,is passed into the upper end of unit 15 above liquid distributing plate27. The evolved gaseous phase component of stream 26, together withstripped off-gas produce as will appear infra, is removed from the topof unit 15 via stream 28, which is processed for recycle of componentsas will appear infra.

The liquid phase flows downwards from plate 27 through and across trays29, which are similar in configuration and function to the trays 18described supra. A rising hot gaseous phase flows upwards through trays29 and decomposes and strips ammonium carbamate from the downflowingliquid phase. The resulting liquid phase of depleted ammonium carbamatecontent collects on liquid retention plate 30, and the liquid on plate30 consists essentially of an aqueous urea solution containing only aminor residual proportion of ammonium carbamate and free ammonia. Plate30 is similar in configuration and function to plate 19 described supra,and plate 30 collects downflowing liquid while permitting upwardsgaseous flow. The collected liquid phase is removed from plate 30 viastream 31, which is heated in heat exchanger 32 by indirect heatexchange with steam or other hot fluid, to decompose residual ammoniumcarbamate. The resulting hot process stream 33, now at an elevatedtemperature typically in the range of 110 to C. and containing anevolved gaseous phase, is passed into unit below plate 30. The evolvedgaseous phase of stream 33 rises through plate and trays 29, and stripsammonium carbamate from the liquid phase. The resultant off-gas passesabove plate 27 and is removed via stream 28.

The residual liquid component of stream 33 collects in unit 15 abovepartition 34 and is removed via stream 35, which consists essentially ofan aqueous urea solution containing a minor residual proportion ofammonia and carbon dioxide, and which is now processed in accordancewith the present invention. Stream 35 is passed through pressurereducing valve 82, and the resultant stream 83 of reduced pressure andtemperature is passed into heat exchange concentrator 36 and is heatedin the tubes 37 of unit 36 in accordance with the present invention,with concomitant evaporation of water and evolution of a gaseous phaseprincipally consisting of water, together with minor proportions ofammonia and carbon dioxide. The resultant mixed vapor-liquid stream 38discharged from the tubes 37 of unit 36 is passed into vapor-liquidseparator 39, from which the product concentrated aqueous liquid ureasolution or melt is removed via stream 40 and passed to suitablefinishing operations such as further evaporative concentration andcrystallization or prilling.

A vacuum effect is maintained in unit 39, with a reduced internalpressure generally in the range of 0.2 to 0.8 kg./sq.cm. within unit 39,by withdrawing stream 41 under vacuum. Stream 41 generally consistsmostly of water vapor, together with minor proportions of ammonia andcarbon dioxide, and stream 41 is passed into flash gas condenser 42which is maintained at a reduced temperature generally in the range of30 to 60 C. by indirect heat exchange with cooling water, brine, or thelike. A vacuum effect is maintained in unit 42 via stream 43, whichpasses to flash gas ejector 44, through which high pressure steam stream45 is passed to provide a venturi stream jet effect, with dischargestream 46 passing to atmosphere.

Returning to unit 42, a condensed vapor receiver 47 is provided forrecovery of liquid condensate from unit 42. The liquid condensate,consisting of a very dilute aqueous solution containing ammonia values,is removed from unit 47 via stream 48, and a portion of stream 48 ispreferably passed via stream 49 to combine with stream 28 and form themixed gasliquid stream 50. The balance of stream 48 passes via stream 51to the ammonia recovery stripper 52, in which the liquid flows downwardsacross and through the plates or trays 53, countercurrent to a risingstripping steam stream 54 which is passed into unit 52 below trays 53.The rising steam strips ammonia and carbon dioxide from the liquidphase, and a stripped liquid water phase is removed from the bottom ofunit 52 via stream 55, which may be discharged to waste, generally afterindirect heat exchange with stream 51, not shown. The stripped ammonia,carbon dioxide and steam vapors are removed from the top of unit 52 viastream 56, which is combined with stream to form stream 57. The stream57 is a combined aqueous vapor-liquid stream containing substantialproportions of ammonia and carbon dioxide derived from stream 28, andstream 57 is now processed to form the aqueous absorbent solutionutilized in accordance with the present invention.

Stream 57 is now passed into cooler-condenser 58, and the process streamis cooled and condensed to liquid in unit 58 by indirect heat exchangewith cooling water or the like. The resultant liquid stream 59, nowcontaining only a minor proportion of gaseous components such as inerts,is passed into separator 60, in which the non-condensable componentsrise countercurrent to wash water stream 61. lnerts or the like, nowsubstantially free of ammonia values, are removed from the top of unit60 via stream 62, which is vented to atmosphere via vent valve 63 asstream 64. The liquid phase in unit 60 is withdrawn via stream 65, whichis employed as an aqueous absorbent solution in accordance with thepresent invention.

Stream 65 is now combined and reacted with off-gas streams derived fromthe process, which results in absorption of ammonia and carbon dioxideinto the liquid phase, formation of ammonium carbamate, and thegeneration of large amounts of heat, due to absorption of the gaseousphase into the liquid phase. The off-gas streams are derived from unit15, with off-gas stream 66 being withdrawn from the middle section ofunit 15 as the off-gas component evolved on the reduction of thepressure of stream 14. Off-gas stream 67 is removed from unit 15 belowpartition 16, and stream 67 is derived from ammonium carbamatedecomposition and oil"- gas evolution in stripping section 18. Streams66 and 67 are preferably combined to form stream 68, which is nowcombined with stream 65 to form a hot reacting gas-liquid mixture stream69, which is passed into the shell of unit 36 external to the tubes 37and serves to heat stream 83 as described supra. Stream 69 is therebycooled in unit 36, and the resultant cooled stream 70 discharged fromthe shell of unit 36 is now processed to produce a concentrated aqueousammonium carbamate solution for recycle to urea synthesis.

Stream 70 is now passed into suitable means for condensation andformation of an aqueous ammonium carbamate solution. In this preferredembodiment of the invention, stream 70 is passed into acondenser-stripper in which substantially pure gaseous ammonia as wellas aqueous ammoniacal ammonium carbamate solution are prepared forrecycle. Stream 70 thus passes into condenser-stripper unit 71 below thepacked gasliquid contact section 72. Liquid ammonia stream 73 is sprayedinto unit 71 below section 72 and vaporizes to provide a cooling effectand also to induce carbon dioxide condensation as ammonium carbamate.Similarly, liquid ammonia stream 74 is sprayed into unit 71 abovesection 72. A recycle aqueous ammonium carbamate solution stream 75 issprayed into bed 72 and flows downwards countercurrent to the risinggaseous phase, and scrubs carbon dioxide from the gas phase into theliquid phase as condensed ammonium carbamate. The resulting concentratedaqueous ammoniacal ammonium carbamate solution collects in the bottom ofunit 71, and is removed via stream 76, with a portion of stream 76 beingdirectly derived as the liquid phase of stream 70. Stream 76 is dividedinto recycle stream 75 and stream 9, which is recycled to urea synthesisas described supra.

The rising gaseous phase in unit 71 above bed 72 now flows upwardsthrough gas reflux section 77, which is provided with a plurality oftrays or bubble cap plates or the like for gas reflux and cooling, tocondense substantially all of the carbon dioxide from the gas phase.Liquid ammonia stream 78 is flashed or vaporized into unit 71 abovetrays 77, and provides a cooling effect as well as final carbon dioxidecondensation to the liquid phase, which is formed on the upper trays byinjecting a small amount of water into unit 71 via stream 79 above trays77. The resultant purified ammonia vapor is removed from the top of unit71 above section 77 via stream 80, which is now condensed to liquidammonia in the cooler-condenser 81 and recycled to urea synthesis viastream 6 as described supra.

Numerous alternatives within the scope of the present invention willoccur to those skilled in the art, besides those alternatives mentionedsupra. The ranges of process variables such as pressure and temperatureenumerated supra constitute preferred embodiments for optimumutilization of the process concepts of the invention, and the processmay be practiced outside of these ranges in suitable instances. Stream66 is preferably produced by substantially adiabatic expansion of stream12 through valve 13, and in many instances stream 66 will consist mostlyof ammonia vapor. In such cases, stream 66 may alternatively be passeddirectly into unit 71 together with streams 70 and 73, preferably afterpreliminary cooling. In this case, stream 68 would be derived solelyfrom stream 67. In some instances, it may be advantageous to include asmall proportion of urea in stream 65, such as by adding the motherliquor from a urea crystallizer to stream 65, in order to promoteammonium carbamate solubility in the system. All of stream 48 may beprocessed via stream 51 in some cases, with j. passing the balance ofsaid withdrawn aqueous solution to stream 49 being omitted and stream 57being derived solely urea synthesis as said recycled aqueous ammoniumcarfrom streams 28 and 56. Vent gases from the ammonia storage bamatesolution, vessels may be added to streams 56 and/or 41. Stream 70 may k.further cooling and refluxing the rising gaseous phase be optionallyadditionally passed through optional crystallizer 5 from said zone in agas reflux zone into which liquid amheat exchange unit 84, for furthercooling by indirect heat monia and water are injected, exchange withcrystallizer mother liquor crystal Slurry of l. recovering substantiallypure ammonia vapor above said the like, prior to passing stream 70 intounit 71. gas reflux Zone, and

An example of?!" industrial application of the Process of the in.recovering urea from said aqueous urea solution of step presentinvention will now be described. (e

EXAMPLE 2. The process of claim 1, in which said synthesis efiluentstream is heated at reduced pressure by passing said synthesis Theinvention was pp to the d g of 200 tons P y effluent stream downwardsthrough a heating zone and counurea production facility. Following areoperating conditions 1 5 tercurrent to a rising hot gaseous phase, andheating the and component flow fates for Principal ProcessStreamsresulting liquid efi'luent phase of reduced ammonium carba-Component flow rate, kg./hr. Pressure,

Temp, g. Carbon 0. sq. cm. Ammonia dioxide Water Urea Inerts 37 19. 3 4,350 42 1.2 5, 595 37 19. 7 37 3. 6 87 21. 8 3, 250 (ii) 230 06 230 5,595 192 230 3, 270 120 22. 2 555 160 22. 2 321 126 22. 2 2, 390 131 3. 957 108 3. 9 264 96 3. 2 96 0. 5 37 45 0. 4 37 v 45 3. 9 29 91 3. 9 8 1403. 9- 110 3. 9 8

' Combined total of streams 73, 74 and 78.

Iclaim: v 1. A recess for the s nthesis of urea from ammonia and Icarbon gioxide which col-{prises 3. The process of claim 2, in whichsaid resulting liquid efa' reacting ammonia, carbon dioxide and-recycledaqueous fluent phase is heated to generate said hot gaseous phase bymates ttat f ss s ete eid h 8 Pt"???- ammonium carbamate solution atelevated temperature wlthdmwfng resultmg q f' ettiluent P1131Se ffcfm htand pressure, to synthesize urea and form a synthesis eftom of 531dhefmng zoftev heat-mg 831d q f phase fluent stream containing urea,ammonium carbamate, exexternal to Said heatmg P sftptftatmg thetesuttlftg hot cess ammonia and water, gaseous phase from the residualliquid phase, and passing the b. heating said synthesis effluent streamat reduced pressure resultmg hot gaseous Phase upwards through i hea ingto decompose ammonium carbamate and generate a first 9 off-gascontaining ammonia, carbon dioxide and water 4. The process of claim 1,in which said aqueous absorbent vapor, solution is an aqueous solutioncontaining dissolved ammonia c. separating said first off-gas from theresidual liquid phase, carbon dioxide.

d. heating said residual liquid phase to generate a second off-gascontaining ammonia, carbon dioxide and water olmionrpomprisesanaqueousurea Sohmonvapor and to form an aqueous urea solution substantially g h-1 r a free of ammonium carbamate and ammonia, 15recess 0 6 am i Winch531d q e a sor ent eabsorbingvat least a portion ofsaid first off gas inan aque solution is formed by at least partially condensing said secondous absorbent solution to form ammonium carbamate in ofitga? of 'P (d)to t' aqtleous q and solution and generate heat while in indirect heatrecycling at least a portion of said aqueous liquid condensate exchangewith said aqueous urea solution, whereby water. as te flt fi ebeq esm 95. The process of claim 1, in which said aqueous absorbent vapor isevaporated from said aqueous urea solution, 7. A process for thesynthesis of urea from ammonia and f. passing the resultant aqueousabsorbent solution and carbon dioxide which comprises residual off-gasbelow a gas-liquid C nta t Zone, whereby a. reacting ammonia, carbondioxide and recycled aqueous said residual off-gas rises through saidzone, ammonium carbamate solution at elevated temperature g. injectingliquid ammonia above n b l Said 20116, and pressure, to synthesize ureaand form a synthesis efwhereby said liquid ammoni vaporiz and arbondioxfluent stream containing urea, ammonium carbamate, ex-

ide is condensed to ammonium carbamate within said cess ammonia andwater,

zone, b. reducing the pressure of said synthesis effluent stream, h.withdrawing aqueous ammoniacal ammonium carbamate whereby a 5 flL isgenerated,

S I 'I below Said Zone, c. separating said first off-gas from theresidual synthesis efi. recycling a portion of the withdrawn aqueoussolution flu t Stream,

above Said Zone, whereby Said 501mm" scrubs the rising d. heating saidresidual synthesis effluent stream to decomoff'gas Phasein Said Zonepose ammonium carbamate and generate a second offe. separating saidsecond off-gas from the residual liquid heating said residual liquidphase to generate a third offgas containing ammonia, carbon dioxide andwater vapor and to form an aqueous urea solution substantially free ofammonium carbamate and ammonia, absorbing at least a portion of saidfirst off-gas and said second ofi gas in an aqueous absorbent solutionto form ammonium carbamate in solution and generate heat while inindirect heat exchange with said aqueous urea solution, whereby watervapor is evaporated from said aqueous urea solution,

h. passing the resultant aqueous absorbent solution and residual off-gasbelow a gas-liquid contact zone, whereby said residual off-gas risesthrough said zone,

. injecting liquid ammonia above and below said zone, whereby saidliquid ammonia vaporizes and carbon dioxide is condensed to ammoniumcarbamate within said zone,

j. withdrawing aqueous ammoniacal ammonium carbamate solution below saidzone,

k. recycling a portion of the withdrawn aqueous solution above saidzone, whereby said solution scrubs the rising off-gas phase in saidzone,

I. passing the balance of said withdrawn aqueous solution to ureasynthesis as said recycled aqueous ammonium carbamate solution,

in. further cooling and refluxing the rising gaseous phase from saidzone in a gas reflux zone into which liquid ammonia and water areinjected,

n. recovering substantially pure ammonia vapor above said gas refluxzone, and

o. recovering urea from said aqueous urea solution of stop 8. Theprocess of claim 7, in which the pressure of said synthesis effluentstream is reduced according to step (b) under substantially adiabaticconditions.

9. The process of claim 7, in which said residual synthesis effluentstream is heated at reduced pressure in step (d) by passing saidresidual synthesis effluent stream downwards through a heating zone andcountercurrent to a rising hot gaseous phase, and heating the resultingliquid effluent phase of reduced ammonium carbamate content to generatesaid hot gaseous phase.

10. The process of claim 9, in which said resulting liquid effluentphase is heated to generate said hot gaseous phase by withdrawing saidresulting liquid effluent phase from the bottom of said heating zone,heating said withdrawn liquid phase external to said heating zone,separating the resulting hot gaseous phase from the residual liquidphase, and passing the resulting hot gaseous phase upwards through saidheating zone.

11. The process of claim 7, in which said aqueous absorbent solution isan aqueous solution containing dissolved ammonia and carbon dioxide.

12. The process of claim 7, in which said aqueous absorbent solutioncomprises an aqueous urea solution.

13. The process of claim 7, in which said aqueous absorbent solution isformed by at least partially condensing said third off-gas of step (f)to form an aqueous liquid condensate, and recycling at least a portionof said aqueous liquid condensate as said aqueous absorbent solution.

2. The process of claim 1, in which said synthesis effluent stream isheated at reduced pressure by passing said synthesis effluent streamdownwards through a heating zone and countercurrent to a rising hotgaseous phase, and heating the resulting liquid effluent phase ofreduced ammonium carbamate content to generate said hot gaseous phase.3. The process of claim 2, in which said resulting liquid effluent phaseis heated to generate said hot gaseous phase by withdrawing saidresulting liquid effluent phase from the bottom of said heating zone,heating said withdrawn liquid phase external to said heating zone,separating the resulting hot gaseous phase from the residual liquidphase, and passing the resulting hot gaseous phase upwards through saidheating zone.
 4. The process of claim 1, in which said aqueous absorbentsolution is an aqueous solution containing dissolved ammonia and carbondioxide.
 5. The process of claim 1, in which said aqueous absorbentsolution comprises an aqueous urea solution.
 6. The process of claim 1,in which said aqueous absorbent solution is formed by at least partiallycondensing said second off-gas of step (d) to form an aqueous liquidcondensate, and recycling at least a portion of said aqueous liquidcondensate as said aqueous absorbent solution.
 7. A process for thesynthesis of urea from ammonia and carbon dioxide which comprises a.reacting ammonia, carbon dioxide and recycled aqueous ammonium carbamatesolution at elevated temperature and pressure, to synthesize urea andform a synthesis effluent stream containing urea, ammonium carbamate,excess ammonia and water, b. reducing the pressure of said synthesiseffluent stream, whereby a first off-gas is generated, c. separatingsaid first off-gas from the residual synthesis effluent stream, d.heating said residual synthesis effluent stream to decompose ammoniumcarbamate and generate a second off-gas, said second off-gas containingammonia, carbon dioxide and water vapor, e. separating said secondoff-gas from the residual liquid phase, f. heating said residual liquidphase to generate a third off-gas containing ammonia, carbon dioxide andwater vapor and to form an aqueous urea solution substantially free ofammonium carbamate and ammonia, g. absorbing at least a portion of saidfirst off-gas and said second off-gas in an aqueous absorbent solutionto form ammonium carbamate in solution and generate heat while inindirect heat exchange with said aqueous urea solution, whereby watervapor is evaporated from said aqueous urea solution, h. passing theresultant aqueous absorbent solution and residual off-gas below agas-liquid contact zone, whereby said residual off-gas rises throughsaid zone, i. injecting liquid ammonia above and below said zone,whereby said liquid ammonia vaporizes and carbon dioxide is condensed toammonium carbamate within said zone, j. withdrawing aqueous ammoniacalammonium carbamate solution below said zone, k. recycling a portion ofthe withdrawn aqueous solution above said zone, whereby said solutionscrubs the rising off-gas phase in said zone, l. passing the balance ofsaid withdrawn aqueous solution to urea synthesis as said recycledaqueous ammonium carbamate solution, m. further cooling and refluxingthe rising gaseous phase from said zone in a gas reflux zone into whichliquid ammonia and water are injected, n. recovering substantially pureammonia vapor above said gas reflux zone, and o. recovering urea fromsaid aqueous urea solution of stop (g).
 8. The process of claim 7, inwhich the pressure of said syNthesis effluent stream is reducedaccording to step (b) under substantially adiabatic conditions.
 9. Theprocess of claim 7, in which said residual synthesis effluent stream isheated at reduced pressure in step (d) by passing said residualsynthesis effluent stream downwards through a heating zone andcountercurrent to a rising hot gaseous phase, and heating the resultingliquid effluent phase of reduced ammonium carbamate content to generatesaid hot gaseous phase.
 10. The process of claim 9, in which saidresulting liquid effluent phase is heated to generate said hot gaseousphase by withdrawing said resulting liquid effluent phase from thebottom of said heating zone, heating said withdrawn liquid phaseexternal to said heating zone, separating the resulting hot gaseousphase from the residual liquid phase, and passing the resulting hotgaseous phase upwards through said heating zone.
 11. The process ofclaim 7, in which said aqueous absorbent solution is an aqueous solutioncontaining dissolved ammonia and carbon dioxide.
 12. The process ofclaim 7, in which said aqueous absorbent solution comprises an aqueousurea solution.
 13. The process of claim 7, in which said aqueousabsorbent solution is formed by at least partially condensing said thirdoff-gas of step (f) to form an aqueous liquid condensate, and recyclingat least a portion of said aqueous liquid condensate as said aqueousabsorbent solution.